In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the traditional multiple access schemes for delivering simultaneous services to a number of terminals. The basic idea underlying the FDMA and TDMA systems is sharing the available resource in such a way that several terminals can operate simultaneously without causing interference. For an FDMA system several frequencies are shared, and for a TDMA system several time intervals are shared.
Telephones operating according to the GSM standard belong to the FDMA and TDMA systems in the sense that transmission and reception are performed at different frequencies and also at different time intervals.
In contrast to these systems using frequency division or time division, the CDMA (Code Division Multiple Access) systems allow multiple users to share a common frequency and a common time channel by using coded modulation. CDMA systems include the CDMA 2000 system, the WCDMA system (Wide Band CDMA) and the IS-95 standard.
In CDMA systems, as is well known to the person skilled in the art, a scrambling code is associated with each base station, and this makes it possible to distinguish one base station from another. Furthermore, an orthogonal code, known by the person skilled in the art as an OVSF code, is allotted to each remote terminal (such as for a cellular mobile telephone, for example). All the OVSF codes are mutually orthogonal, thus making it possible to distinguish one remote terminal from another.
Before sending a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the remote terminal. In CDMA systems, it is again possible to distinguish between those which use a distinct frequency for transmission and reception (CDMA-FDD system) and those which use a common frequency for transmission and reception, but distinct time domains for transmission and reception (CDMA-TDD system).
The invention applies advantageously to communication systems of the CDMA type, and more particularly to systems of the CDMA-FDD type. The invention applies also to communication systems of the FDMA and TDMA type, in particular to GSM and GPRS telephones, and more generally to terminals operating according to the UMTS standard which must be capable of operating both under a CDMA system, like the WCDMA system, and under the FDMA and TDMA systems, for example.
In remote terminals, such as cellular mobile telephones, there is currently provided a single power amplifier for transmission. This power amplifier has a wide radio frequency power operating range. Also, in CDMA-FDD systems, the power amplifier is operating continuously during the communications.
Moreover, the transmission power delivered by the power amplifier can vary within a predetermined range of powers, typically from −50 dBm to 24 dBm for third-generation mobile telephones. In this power range, the transmission power is adjusted as a function of power information received regularly by the telephone and originating from the base station.
At present, the power amplifier is designed in such a way as to exhibit the greatest effectiveness for the maximum transmission power. For intermediate or low power, there is significant deterioration in the effectiveness since the quiescent current of the power amplifier does not change, while the power transmitted decreases. Thus, in these modes of operation at low or intermediate power, the effectiveness, that is, the efficiency, drastically decreases to less than a percent. This results in a loss of energy at the level of the battery, thereby reducing its lifetime.